1. Field of the Invention
The present invention relates to a semiconductor device equipped with MOS (Metal Oxide Semiconductor) transistors with different drain breakdown voltages that are provided on the same SOI (Silicon On Insulator) substrate.
2. Description of the Related Art
In recent years, mobile electronic appliances have been made smaller and lighter, making it essential to miniaturize ICs that are mounted in such electronic appliances. In particular, for an electronic appliance on which a liquid crystal display device is mounted, there are strong demands to reduce the chip area of a driver IC, which drives the liquid crystal display device, by mounting transistors with a low breakdown voltage for low voltage operations and transistors with a high breakdown voltage for high voltage operations on a same substrate (i.e., in the same chip).
However, to achieve a sufficiently high breakdown voltage, it is not possible to reduce the area of a transistor forming region for high breakdown voltage transistors. Accordingly, no matter how much low breakdown voltage transistors are miniaturized, it has not been possible to make large reductions in the chip area of a driver IC.
When high breakdown voltage transistors and low breakdown voltage transistors are formed on the same SOI substrate, it is necessary to change the structures of the transistors to make the transistors suitable for an SOI substrate so as to avoid the substrate floating effect and the like that are characteristic to SOI substrates. Therefore, it is not possible to apply design resources that have been produced for the case where transistors are formed on a bulk silicon substrate.